Modelling the effects of changing discharge patterns on lake phytoplankton communities

Lead Research Organisation: NERC Centre for Ecology and Hydrology
Department Name: Parr

Abstract

In the future it has been predicted that the climate will change somewhat. Included in these predictions is the idea that rainfall patterns will not be as they are now, but rather that there might be more rain in the winter and less in the summer. As rainfall determines the amount of water flowing into a given lake, the length of time water typically stays in a given lake, known as the residence time, may well also change in the future, at least for lakes with relatively short (say, less than 100 days) residence times. Previous work has shown that the length of this residence time can be very important for determining the amount of phytoplankton / microscopic plants that live in a water environment / that can exist in the lake. The residence time has also been shown to affect the types of phytoplankton, for example whether they are small green algae or toxic blue-green algae, that exist and the timing of when they grow. This residence time can affect the phytoplankton because it is one of the controls on the amount of essential nutrients for the phytoplankton that enter and leave a lake. The way that these nutrients enter the lake is very important as, if they arrive through the inflow, then an increase in inflow leads to more nutrients arriving and more nutrients leaving the lake. If, however, the nutrients arrive in the lake from point sources such as sewage works, then an increase in flow only has the effect of removing more nutrients from the lake. Conversely this means that a decrease in the flow will increase the amount of these essential nutrients that stay in the lake. This is all very important as the amount and the type of phytoplankton in a lake controls the quality of the water in the lake. We need this water quality to be of a high standard for it to be safe to drinking and for recreational activities around a lake, but also to meet legal requirements set be the European Union. There is therefore a possibility that just by a change in rainfall patterns, with no other changes taking place to a lake, that water quality will become poorer in some short residence time lakes. The intention here is to examine how these changes in flow can affect a lake system by using a computer model. This model simulates the temperatures of a lake from given weather data and also the type of phytoplankton that can grow in the lake from data on the nutrients entering the lake, the temperature of the lake and the amount of light that the phytoplankton receive. By running this model thousands of times, each time using slightly different information on the daily discharge, (that is, the inflowing and outflowing water for each day) and the source of nutrients to the lake we can study how a lake will respond to these sorts of changes. We will study two different lakes with two different typical residence times to ensure we have a good overview of the likely response of lakes to changing rainfall patterns and whether particular times of year are more important then others.
 
Description Algal community composition is altered by changes in flow patterns with decreased flow in the summer leading to increases in cyanobacteria abundance. Flow patterns were generated which maintained a uniform annual discharge rate but incorporated a continuous shift from constant flow to more hibernal or more aestival flow. Fig. 1b) demonstrates that, assuming a point source load, as flow shifts to being less in the summer (longer residence times) the percentage of cyanobacteria in Esthwaite Water increases. This culminates in nearly 70 % more cyanobacteria in the summer than the scenario with constant flow throughout the year.
Flow patterns also induce large changes to the timing of vernal and autumnal algal blooms (of order 50 days) although these changes are most noticeable at very short residence times, suggesting the main 'flow' driver of phenological change is direct flushing of algae rather than systematic changes to in-lake nutrients. Other, non-discharge related, parameters also affect phenology and these influence the periods when flow is most important.
Exploitation Route In understanding the impacts of:
Climate change
Eutrophication
Sectors Environment